The invention relates to a method and device for detecting phase failures, in particular phase failures in the form of line failures, in a converter, in particular in a converter connected to a supplying line.
To protect expensive workpieces during the production process in a machine tool, phase failures, in particular line phase failures, must be detected quickly and reliably and the equipment placed e.g. in a safe operating state on the basis of such detection. This is in order to protect the workpiece currently being processed e.g. in a multi-axis lathe/milling machine from damage caused by unwanted/uncoordinated movement of the machine axes due to a power supply dip. Because the value of machined workpieces is sometimes in the six-figure or even in the seven-figure range, a high outlay for protecting such workpieces is justified.
The approach described in the following is suitable for use in machine tools, particularly metal-cutting machine tools, and other production machines. The term machine tool commonly refers to all machines which are used among other things in mechanical engineering and toolmaking for processing workpieces using tools. On the other hand, an industrial robot is an all-purpose, programmable production machine which is designed and set up not only to process workpieces but alternatively also to handle workpieces and for assembly purposes. Here and in the following, the expression machine tool is used generically for terms such as machine tool, production machine, industrial robot and the like.
If an impending line failure or disturbance is suspected, precautionary retracting movements of the machine axes of a machine tool shall be initiated. The overall concept of the machine tool must be geared to this requirement. Machine tool manufacturers demand corresponding functions from the integrated drive and control units.
In such applications, a self-commutated IGBT bridge converter is typically used as a dynamic line inverter with feedback capability. The converter controls the DC link voltage of the drive group and impresses (e.g. using pulse width modulation PWM) sinusoidal line currents which result in the necessary power flow to the line. In order to keep the pulse frequency components of the converter output voltage away from other line users, line filters are used which typically contain inductors and at least one capacitor in the filter branch. The converter is typically controlled such that the reactive power demand at the connection point of the filter is virtually zero, i.e. apart from the higher-frequency distortion components, pure active current is exchanged with the line (displacement factor cos φ=1). The reactive power demand of the capacitor in the line filter is provided for this purpose by the converter.
The comparatively frequently occurring event of a line voltage dip or of a short circuit or ground fault does not need to be considered in the following, as the associated large voltage difference between inverter and dipped supply voltage results in a large current which can be easily detected. In addition, large variations in the voltage amplitude can be easily detected using voltage sensing and/or model calculations of the connection voltage.
On the other hand, a different situation arises in the event of a high-resistance failure of a line phase. A typical example of this is the tripping of a cutout in a line phase. The voltage of the lost line phase is retained and simulated by the capacitor of the line filter required for operating the converter. Particularly under no-load conditions, the detection of a high-resistance line failure therefore becomes a problem for which a solution is proposed here. On the basis of the thus possible detection of a high-resistance supply failure, a machining step can be preventing from starting in the machine tool, for example, if a branch cutout has previously tripped.
The high-resistance failure of more than one line phase is—because of interrupted energy flow of the 3-conductor connection—once again insignificant and will not therefore be considered here. For example, such a situation could be detected from the interrupted current flow or from the abruptly changing voltage at the line filter.
Although the approach presented here is explained taking the example of a line-side use of a PWM inverter, it is equally applicable to motor-side inverters with sine wave output filter (having longitudinal inductance and transverse capacitance). The topology and the control methods can be used in a basically similar manner, merely replacing the block “line” by an electrical motor, in particular a synchronous motor. This can be explained by the fact that line supplies are typically created using electrical generators, which means that, even in the case of a line inverter, the block “line” can consist of an electrical machine.
An object of the present invention is to specify a method for detecting phase failures, in particular phase failures in the form of single-phase line phase interruptions hereinafter referred to a line faults for short, in particular for detecting line faults during no-load or part-load operation of active line inverters with line filters.